Approximately 42 percent of all West Virginians rely on groundwater for their domestic water supply. However, prior to 2008, the quality of the West Virginia's groundwater resource was largely unknown. The need for a statewide assessment of groundwater quality prompted the U.S. Geological Survey (USGS), in cooperation with West Virginia Department of Environmental Protection (WVDEP), Division of Water and Waste Management, to develop an ambient groundwater-quality monitoring program. The USGS West Virginia Water Science Center sampled 300 wells, of which 80 percent were public-supply wells, over a 10-year period, 1999-2008. Sites for this statewide ambient groundwater-quality monitoring program were selected to provide wide areal coverage and to represent a variety of environmental settings. The resulting 300 samples were supplemented with data from a related monitoring network of 24 wells and springs. All samples were analyzed for field measurements (water temperature, pH, specific conductance, and dissolved oxygen), major ions, trace elements, nutrients, volatile organic compounds, fecal indicator bacteria, and radon-222. Sub-sets of samples were analyzed for pesticides or semi-volatile organic compounds; site selection was based on local land use. Samples were grouped for comparison by geologic age of the aquifer, Groups included Cambrian, Ordovician, Silurian, Devonian, Pennsylvanian, Permian, and Quaternary aquifers. A comparison of samples indicated that geologic age of the aquifer was the largest contributor to variability in groundwater quality. This study did not attempt to characterize drinking water provided through public water systems. All samples were of raw, untreated groundwater. Drinking-water criteria apply to water that is served to the public, not to raw water. However, drinking water criteria, including U.S. Environmental Protection Agency (USEPA) maximum contaminant level (MCL), non-enforceable secondary maximum contaminant level (SMCL), non-enforceable proposed MCL, or non-enforceable advisory health-based screening level (HBSL), were used as benchmarks against which to compare analytical results. Constituent concentrations were less than the MCLs in most samples. However, some samples exceeded non-enforceable SMCLs, proposed MCLs, or advisory HBSLs. Radon-222 concentrations exceeded the proposed MCL of 300 pCi/L in 45 percent of samples, and iron concentrations exceeded the SMCL of 300 ug/L in 57 percent of samples. Manganese concentrations were greater than the SMCL (50 ug/L) in 62 percent of samples and greater than the HBSL (300 ug/L) in 25 percent of the samples. Other sampled constituents, including organic compounds and trace elements, exceeded drinking-water criteria at much lower frequencies. The radon-222 median concentrations in samples from Cambrian, Ordovician, Silurian, Permian, and Quaternary aquifers exceeded the proposed 300 pCi/L MCL. Although median radon concentrations for wells in Devonian, Mississippian, and Pennsylvanian aquifers were less than the proposed MCL, radon concentrations greater than the proposed MCL were measured in samples from aquifers of all geologic ages. The median iron concentrations for samples from Devonian and Pennsylvanian aquifers were greater than the 300 ug/L SMCL. Iron concentrations exceeded the SMCL in aquifers of all geologic ages, except Cambrian. Median concentrations of manganese exceeded the SMCL in samples from Devonian, Pennsylvanian, and Quaternary aquifers. As with iron, manganese concentrations were found to exceed the SMCL in at least one sample from aquifers of all geologic ages, except Cambrian. Pesticides were detected most frequently and in higher concentrations in limestone-dominated areas. Most of West Virginia's agriculture is concentrated in those areas. This study, the most comprehensive assessment of West Virginia groundwater quality to date, indicates the water quality of West Virginia's groundwater is generally good; in the ma